This invention relates to an improved process for the treatment of a gas containing hydrogen sulfide and, more particularly, to a method of recovering valuable substances contained in an effluent alkali salt solution diverted from an alkaline wash solution recirculating system in Stretford processes.
The Stretford processes using an alkali vanadate-ADA (sodium salts of the 2,6 and 2,7 isomers of anthraquinone disulfonic acid) system are now widely in operation. In such processes, gas containing hydrogen sulfide is washed with an alkaline wash solution containing sodium carbonate and bicarbonate to absorb the hydrogen sulfide, and the resulting hydrosulfide is oxidized to elemental sulfur by means of an alkali vanadate, with a simultaneous valence change of vanadium from five to four. The elemental sulfur is removed from the wash solution by, for example, floth flotation. The reduced vanadate is reoxidized by aerial oxygen in the presence of ADA. The regenerated wash solution is recycled to the absorption zone.
The chemistry of the process can be represented by the following equations: EQU Absorption: H.sub.2 S + Na.sub.2 CO.sub.3 .fwdarw. NaHS + NaHCO.sub.3 EQU oxidation: HS.sup.- + V.sup.5+ .fwdarw. S + V.sup.4+ EQU regeneration: V.sup.4+ + ADA .fwdarw. V.sup.5+ + ADA (reduced) ADA (reduced) + O.sub.2 .fwdarw. ADA EQU overall equation: H.sub.2 S + 1/20.sub.2 .fwdarw. S + H.sub.2 O
Besides the principal reactions, a number of side reactions occur in the process to form the following inactive salts. EQU HS.sup.- + O.sub.2 .fwdarw. Na.sub.2 S.sub.2 O.sub.3, Na.sub.2 SO.sub.4, Na.sub.2 SO.sub.3
these inactive salts tend to accumulate in the recycled wash solution and excessive accumulation will cause precipitation of these salts together with ADA and vanadium compounds.
To cope with this problem, a portion of the recirculating wash solution is generally discarded and replaced with a fresh solution to control the content of these salts within a predetermined value which is sufficient to keep ADA dissolved in the recycling wash solution.
Such an effluent aqueous alkali salt solution discharged from the circulation system has, in general, the following composition.
______________________________________ ADA 5 - 1 g/l V compd. 5 - 1 Chelating agent 4 - 1 NaHCO.sub.3 5 - 30 Na.sub.2 CO.sub.3 1 - 20 NaSCN 3 - 1 Na.sub.2 SO.sub.4 100 - 250 total 200 - 400 Na.sub.2 S.sub.2 O.sub.3 100 - 300 ______________________________________
Thus, the effluent contains substances useful in the oxidation and regeneration stages, i.e. ADA, vanadium compound, chelating agent, as well as active salts useful in the absorption stage, i.e., sodium carbonate and bicarbonate. Accordingly, it is disadvantageous to discard the effluent.
Several methods have been proposed to recover useful substances from such effluent. For example, in U.S. Pat. No. 3,941,875, there is disclosed a process wherein the effluent is subjected to submerged combustion at high temperature to convert inactive alkali metal salts into active salts. While this process is effective in recovering sodium and vanadium components, it is impossible to recover valuable components such as ADA and chelating agents.
A method is proposed in Japanese unexamined patent application (Tokkyo Kokai) No. 68995/1975, wherein the effluent salt solution is mixed with methanol to recover ADA, vanadium compounds and chelating agent. This method is, however, disadvantageous because a relatively large amount of methanol is consumed.